1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus. More particularly, the present invention relates to an image forming apparatus of the type forming a test patch with a preselected set value and then sensing the amount of toner deposited on the test patch for controlling image density.
2. Description of the Related Art
An electrophotographic image forming apparatus usually includes an image carrier, e.g., a photoconductive element caused to rotate by a motor. While the image carrier is in rotation, a charger uniformly charges the surface of the image carrier to a preselected potential. An exposing unit exposes the charged surface of the image carrier imagewise to thereby form a latent image. A developing device develops the latent image for thereby producing a corresponding toner image. An image transfer unit transfers the toner image to a sheet or recording medium. In a full-color mode, such a process is repeated color by color for forming toner images of different colors on the image carrier one above the other and then transferring the resulting full-color image to a sheet. Alternatively, toner images of different colors may be sequentially formed on the image carrier while being transferred to a sheet one by one.
In a potential control system available with the image forming apparatus, a latent image representative of a patch pattern, or reference latent image, is formed on the image carrier and then developed by the developing device. The control system measures a developing characteristic based on the surface potential of the patch pattern and the amount of toner deposited thereon. The control system then determines, based on the developing characteristic, various potentials including a bias potential for development and a potential to which the image carrier should be charged. For example, a specific potential control system uses a plurality of patch patterns and reference values corresponding one-to-one to the patch patterns and compares each reference value and the amount of toner deposited on a particular patch pattern, thereby determining various potentials. Another specific potential control system senses the surface potentials of patches and the amounts of toner deposited thereon with a sensor and then linearly approximates a developing characteristic by using the resulting data. The system then determines various potentials by using the slope of the linear approximation as a developing efficiency.
However, it is extremely difficult with the potential control system described above to determine a reference value. Particularly, when a developer used is noticeably susceptible to environment or aging, the algorithm used to control various potentials becomes difficult because the influence of the varying environment or agent should be avoided. As a result, an extremely long period of time is necessary for the potentials to become stable. The control system of the type relying on linear approximation fails to achieve sufficient accuracy against the variation of the developer and that of the image carrier, resulting in unstable potential control. This is particularly true when such a control system is applied to a full-color copier extremely susceptible to potential variation; stability is short in the highlight portion of a full-color image among others.
A current trend in the imaging art is toward an electrophotographic image forming apparatus not including a potential sensor. This is directed toward cost reduction. Moreover, recent control over the quantity of exposing light is shifting from multilevel control (e.g. 256 tones) to two-level or four-level control, preventing latent image control using a potential sensor from being fed back to the quantity of light.
The control using such a small number of levels is implemented by the recent resolution as high as 600 dpi (dots per inch) or 1,200 dpi, which is far greater than the conventional 300 dpi or 400 dpi. The high resolution reduces the size of a single dot and therefore allows halftone to be rendered without resorting to delicate control over the quantity of light. Further, in parallel with an increase in the number of prints from the order of several prints to the order of several ten prints, the load that a CPU (Central Processing Unit) bears is increasing. The control using a small number of levels serves to reduce the load on the CPU.
Under the above circumstances, development potential control, which is the extension of the traditional potential control, is predominant as control of the type using a sensor responsive to the amount of toner deposition. Generally, the development potential control forms a number of patches by varying a development potential, which is a difference between a bias for development and the surface potential of an image carrier. A photosensor senses the amount of toner deposited patch by patch. The sensed amounts of toner are used to determine a relation between the development potential and the amount of toner deposition. This relation is, in turn, used to determine the conditions of an image forming apparatus. Consequently, the characteristic of the apparatus is produced in the form of scattered values each corresponding to a particular patch. The scattered values are subjected to linear approximation for determining a development potential that implements a target amount of toner deposition. In practice, a development bias, a charge potential and a quantity of light, for example, are determined that control the development potential.
A sensor using diffuse reflection light has been proposed for the above-described density control of the type using a plurality of patches. This kind of sensor is capable of sensing the amount of toner deposition, i.e., image density with high accuracy.
The multi-point type of density control stated above has a problem that it must form a number of patches with different development potentials. Another problem is that the calculations including the linear approximation extend a period of time necessary for control. Although the diffuse reflection type of sensor may make up for the short accuracy of linear approximation, it cannot reduce the processing time. In addition, toner is consumed in an amount corresponding to the number of patches, increasing the running cost of the apparatus.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-302892, 6-148994, 9-319180 and 11-258873.